installer: switch to using Inno Setup's `CloseApplications` directive.

Now the installer will not perform a check when it starts for running process(es). Instead, it will warn just before the actual installation and offer to close any running process(es).

See also http://www.jrsoftware.org/ishelp/index.php?topic=setup_closeapplications and http://www.jrsoftware.org/ishelp/index.php?topic=setupcmdline